Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are usually positioned within the right side of the heart, either within the right ventricle or right atrium, or both, for making electrical contact with their respective heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to sense cardiac electrical activity and deliver the desired therapy.
Traditionally, therapy delivery had been limited to the venous, or right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. If a blood clot were released arterially from the heart left side, as for example the left ventricle, it could pass directly to the brain potentially resulting in a paralyzing or fatal stroke. However, a blood clot released from the right heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain.
Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy to the left heart. These lead structures and methods avoid direct electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus region of the heart. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portions of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.
It has been demonstrated that electrodes placed in the coronary sinus region of the heart may be used for left atrial pacing and sensing, left ventricular pacing and sensing, or cardioversion and defibrillation. These advancements enable implantable cardiac stimulation devices to provide therapy and diagnostics not previously possible. For example, it has been demonstrated that atrial fibrillation may be terminated with the application of electrical energy between an electrode placed in the right atrium and an electrode placed in the coronary sinus adjacent the left atrium. Further, interchamber conduction delays during bi-chamber (both atria or both ventricles either simultaneously or in sequence) pacing may now be monitored between the right ventricle and the left ventricle and/or between the right atrium and the left atrium to determine if normal propagation of R waves and P waves, respectively, is present.
It has recently been hypothesized that dissociation between a right chamber and a corresponding left chamber may be responsible for the precipitation of tachyarrhythmias. More specifically, it has been hypothesized that long or significantly variable interventricular conduction delays or interatrial conduction delays may be prone to the development of ventricular fibrillation or atrial fibrillation, respectively.
A recent study indicated that patients with long interatrial conduction delay were likely to develop atrial fibrillation. The study further indicated that patients who had a history of previous atrial fibrillation had a 90% risk of developing atrial fibrillation after atrial flutter ablation. Atrial ablation is a well known therapy wherein cauterization, cryosurgery, 10 diathermy, fulguration, laser or cutting is applied to an atrium to eliminate the site of origin of atrial tachycardia or to interrupt the pathway through which the atrial tachyarrhythmia travels. The study concluded that patients with a history of previous atrial fibrillation and who have long interatrial conduction delay are unlikely to benefit from atrial flutter ablation.